Amino acid coating of Fe3O4 surfaces under dry and hydrated conditions

This study explores the adsorption behavior of two amino acids (glycine - gly and glutamic acid - glu) as coating molecules for Fe3O4 nanoparticles to improve their biocompatibility for medical applications. Amino acid adsorption on the dry and hydrated reconstructed Fe3O4 (001) surface was investigated by means of quantum mechanical methods. According to Density Functional Theory (DFT) calculations (HSE06), the deprotonated form is favored over the zwitterion for both amino acids on the dry surface (by -0.14 and -0.59 eV for gly and glu, respectively), whereas the order is reversed under hydrated conditions (by +0.07 and +0.35 eV, for gly and glu, respectively). Given the small energy difference in the case of gly, deprotonated and zwitterionic adsorption modes are expected to compete and, probably, to coexist on the magnetite surface both under dry and hydrated conditions. Density Functional Tight-Binding (DFTB) method can only provide a qualitative energy order for amino acids adsorption on the dry magnetite surface due to excessive stabilization of the deprotonated adsorbed form (-0.54 and -1.24 eV for gly and glu, respectively). This overestimation favors the deprotonated form even under hydrated conditions (by -0.41 and -0.18 eV for gly and glu, respectively), which contradicts the more accurate DFT results. The approximate DFTB method presents limitations in correctly capturing the relative stability of different protonation states.